1. Field of the Invention
The present invention relates to a retrofocus type projection lens with high performance suitable as a projection lens of a projection image display apparatus for enlarging and projecting an image on a display screen of a liquid crystal display device or a micro mirror device (DMD), etc., and a projection image display apparatus employing the projection lens.
This application is based on Japanese Patent Applications Nos. 2003-106599 and 2004-014911 filed on Apr. 10, 2003 and Jan. 22, 2004, respectively, the disclosures of which are incorporated herein by reference in their entirety.
2. Description of Related Art
A projection image display apparatus such as a front projection image display apparatus in which a projection lens is positioned at the same side as an observer with respect to a screen and a rear projection image display apparatus in which a projection lens and an observer are positioned with interposing a screen therebetween are known.
Among them, the rear projection image display apparatus receives a light source, a screen, and elements therebetween in a housing in order to reduce influence of external light, and applies the light to the screen at a front surface of the housing from the projection lens at a back surface. Enlargement of a display screen and decrease of the total depth of an apparatus have been required for the rear projection image display apparatus.
For this reason, a configuration capable of downsizing the apparatus by allowing mirrors provided in the housing to reflect the light emitted from the projection lens and thus folding a light path is well known. However, in the rear projection image display apparatus comprising the mirrors in the housing, large-size flat mirrors and high rigid frame for supporting the mirrors are required, thereby causing the cost of an apparatus to go up.
In the rear projection image display apparatus, minimizing the size of a television body is required along with the enlargement of a display screen. In order to satisfy these requirements, it is necessary to increase a field angle of the projection lens and decrease a focal distance of the projection lens.
In the projection lens, since a prism used for color composition or lighting is interposed between the lens and a display device, a long back-focus is required for the lens, and since characteristics of the prism are not varied in the screen, it is required that a reduction side of the lens is telecentric. For this reason, in order to increase the field angle of the projection lens, the projection lens should have a retrofocus type configuration comprising a front group of lenses having a negative power and a rear group of lenses having a positive power, and the powers of the front group of lenses and the rear group of lenses should be enhanced, whereby the symmetry of the lenses about an iris is deteriorated remarkably and correction of various aberrations is made difficult.
Many retrofocus type projection lenses with an aim of increasing the field angle and with a long lens back were conventionally suggested, and are disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2001-42211, Japanese Unexamined Patent Application Publication No. 2003-15033 and Japanese Unexamined Patent Application Publication No. 2003-57540. All of the above mentioned publications are incorporated herein by reference for their helpful background information on previous attempts to generate an aspheric surface quickly and accurately.
However, in the conventional design examples, the field angle in Japanese Unexamined Patent Application Publication No. 2001-42211 is 92°, the field angle in Japanese Unexamined Patent Application Publication No. 2003-15033 is 100°, and the field angle in Japanese Unexamined Patent Application Publication No. 2003-57540 is 81°. As a result, sufficient increase of the field angle has not been accomplished. Therefore, the projection distance is still large, and thus it is not possible to decrease the depth of the projection image display apparatus.